A construction machine including a dozer in the related art, as shown in FIG. 1, is configured so that a large chamber d1 and a small chamber d2 of a hydraulic cylinder d communicate with each other by a pipe h in order to drive a dozer blade that is a working device. In this case, a double check valve is used to prevent neutral oil leakage of the hydraulic cylinder d which may occur when the equipment is fixed to the ground using the dozer blade of the construction machine.
As illustrated in FIGS. 2 and 3, the construction machine that adopts the double check valve in the related art includes a hydraulic pump P; a hydraulic cylinder d connected to the hydraulic pump P to drive a dozer blade f; a control valve a installed in a flow path between the hydraulic pump P and the hydraulic cylinder d and shifted to control a start, a stop, and a direction change of the hydraulic cylinder d; a pair of check valves b and c installed to open and close a flow path between the control valve a and the hydraulic cylinder d and supported by pressure pieces f1 and f2 elastically supported by first and second elastic members e1 and e2; and a double check valve k preventing sinking of the dozer blade f by a plunger h that is slidably installed between the check valves b and c.
The elastic members e1 and e2 may typically be compression coil springs having elastic force against hydraulic pressure.
Referring to FIG. 3, the operation principle of the double check valve k in the related art will be described.
First, in the case where the control valve a is kept in a neutral state, a checking function is performed by the check valves b and c that are elastically supported by the first and second elastic members e1 and e2 and the pressure pieces f1 and f2 if the dozer blade f temporarily descends.
That is, the first flow paths s1 and s3, which make the control valve a and a small chamber d2 of the hydraulic cylinder d communicate with each other, are intercepted by the check valve b, and the second flow paths s2 and s4, which make the control valve a and a large chamber d1 of the hydraulic cylinder d communicate with each other, are intercepted by the check valve c.
Accordingly, hydraulic fluid from the hydraulic pump P is not supplied to the hydraulic cylinder d. Further, hydraulic fluid from the hydraulic cylinder d does not return to a hydraulic tank.
Accordingly, the dozer blade f is prevented from sinking by the weight of the dozer blade itself.
On the other hand, if the control valve a is shifted in the right direction as shown in FIG. 3 by signal pressure or pilot signal pressure that is supplied from an outside, the hydraulic fluid from the hydraulic pump P flows into the first flow path s1 of the double check valve k through the control valve a.
The hydraulic fluid of the first flow path s1 acts on a hydraulic pressure portion n1 to shift the plunger h in the left direction in the drawing, and presses the check valve b in the right direction in the drawing to release the checking function (at this time, the first elastic member e1 receives a compression force). That is, the first flow paths s1 and s3 of the double check valve k communicate with each other.
At this time, due to the shifting of the plunger h, the check valve c is pushed in the left direction in the drawing to release the checking function (at this time, the second elastic member e2 receives the compression force). That is, the second flow paths s2 and s4 of the double check valve k communicate with each other.
Through this, the hydraulic fluid from the hydraulic pump P passes through the control valve a and the first flow paths s1 and s3 of the double check valve k in order and is supplied to the small chamber d2 of the hydraulic cylinder d. At this time, the hydraulic fluid discharged from the large chamber d1 of the hydraulic cylinder d1 passes through the check valve c of which the checking function has been released, the second flow paths s2 and s4 of the double check valve k, and the control valve a in order, and returns to the hydraulic tank.
Accordingly, the hydraulic cylinder d is driven to contract by the hydraulic fluid supplied from the hydraulic pump P.
By contrast, as illustrated in FIG. 3, if the control valve a is shifted in the left direction in the drawing by the signal pressure supplied from the outside, the hydraulic fluid from the hydraulic pump P passes through the control valve a and makes the second flow paths s2 and s4 of the double check valve k communicate with each other. Accordingly, the hydraulic fluid discharged from the small chamber d2 of the hydraulic cylinder d passes through the check valve b of which the checking function has been released, the first flow paths s1 and s3 of the double check valve k, and the control valve a in order, and returns to the hydraulic tank.
Accordingly, the hydraulic cylinder d is driven to expand by the hydraulic fluid supplied from the hydraulic pump P.
However, the double check valve in the related art has the problem that the check valve repeats an abrupt opening and closing operation by an external load to cause the occurrence of noise and hunting phenomenon.
For example, when the hydraulic pressure presses an inlet port of the first flow path s1, the check valve c of the second flow path s4 is opened, and in this case, due to the weight of the working device, the small chamber side of the hydraulic cylinder a temporarily expands at high speed and the large chamber side contracts at high speed.
At this time, the pressure on the side of the second flow path s3 and the first flow path s1 is decreased, and the pressure on the side of the second flow path s4 is increased. Through this, the check valve c is closed by the pressure and the elastic force of the elastic member on the side of the second flow path s4, and thus the operation of the hydraulic cylinder a stops temporarily. Thereafter, since the flow rate is continuously supplied to the side of the first flow path s1 for the operation, the check valve c of the second flow path s4 is opened again and the hydraulic cylinder a starts its operation.
As a result, the double check valve for the construction machine in the related art has the problem that the check valve repeats the opening and closing operation until the equipment stops its operation and thus noise and hunting phenomenon occur frequently.